Solvatochromism of new tetraphenylethene luminogens: integration of aggregation-induced emission and conjugation-induced rigidity for emitting strongly in both solid and solution state

In this study, we synthesized and characterized four tetraphenylethene (TPE) analogs, investigated their photophysical properties, and conducted quantum chemical calculations. Some molecules exhibited aggregation-induced emission enhancement behavior and showed efficient emission in both solid and solution states. Solvatochromism was observed in particular derivatives, with solvent polarity influencing either a bathochromic or hypsochromic shift, indicating the occurrence of photoinduced intramolecular charge transfer (ICT) processes. Quantum chemical calculations confirmed that variations in molecular packing and rigidity among the TPE analogs contributed to their diverse behavior. The study showcases aggregation in luminophores without significant impact on the excited state and highlights how minor alterations in terminal substituents can lead to unconventional behavior. These findings have implications for the development of luminescent materials. Furthermore, the synthesized compounds exhibited biocompatibility, suggesting their potential for cell imaging applications.

4-Bromobenzophenone, 4,4'-dimethoxybenzophenone, and diphenylmethane were purchased from Alfa Aesar.4-Formylphenylboronic acid pinacol ester and p-nitrobenzyl cyanide were purchased from ThermoFisher Scientific.Anhydrous grade solvents were purchased from Sigma-Aldrich and Acros Organics.The purity of the compounds was confirmed using 1 H NMR, 13  Compound 5 was synthesized with minor modifications following the documented procedure.In summary, Diphenylmethane was dissolved in tetrahydrofuran under a nitrogen atmosphere and cooled to 0°C.Slowly, 1.2 equivalents of n-butyl lithium (2.5M in hexanes) were added dropwise to the reaction mixture using a syringe or cannula.The resulting mixture was stirred at the same temperature for 30 minutes, causing it to turn dark brown.Then, 1.1 equivalents of Bromobenzophenone were added gradually to the solution and stirred at room temperature for 12 hours.
The reaction was quenched by adding ice-cold saturated ammonium chloride solution and then extracted with ethyl acetate twice.The organic layer was washed with excess water and brine solution, followed by drying with magnesium sulfate.The solution was filtered and evaporated to obtain the crude product.To purify the crude product, it was suspended in toluene and heated to reflux using a Dean-Stark apparatus for 3 hours while adding 0.5 equivalents of para-toluene sulfonic acid.The reaction mixture was then treated with water and extracted with ethyl acetate twice.The organic layer was washed with excess water and brine solution, dried with magnesium sulfate, filtered, and evaporated to obtain the crude product.The pure product was isolated by recrystallization using methanol, resulting in an off-white crystalline solid with a yield of 80%.

Preparation of 4-(1,2,2-triphenylvinyl)benzaldehyde (A1): (Prepared according to literature) 2
To a stirred solution of 5 (2.00 g, 4.08 mmol) in 50 mL anhydrous THF was added n-BuLi (6.4 ml, 2.5 M in hexane, 10.2 mmol) at -78 o C under nitrogen.The mixture was first stirred for 2 h at this temperature and then warmed to room temperature.After stirring for 1 h, the mixture was cooled again to -78 o C, and N,N-dimethylformamide (1.1 mL, 14.56 mmol) was added in one portion.The solution was stirred overnight and allowed to warm to room temperature gradually.
100 mL of 2 M aqueous hydrochloric acid solution was added to quench the reaction.After stirring for 2 h, the organic layer was separated, and the aqueous layer was extracted with 100 mL DCM three times.The organic layer was dried over MgSO4.After solvent evaporation under reduced pressure, the residue was purified by silica gel column chromatography using hexane/ethyl acetate (4:1 v/v) as eluent to give the desired product as a pale yellow solid in 65% yield (1.1 g).Step 1: A dry three-neck RB flask was filled with magnesium (1.4 g, 62.5 mmol) under a nitrogen atmosphere.Then, 5 mL of tetrahydrofuran (THF) with a small amount of iodine as a catalyst was added and stirred at room temperature for 5 minutes.After that, a solution of 4-methoxyphenyl bromide (9.0 g, 48.1 mmol) in 40 mL of THF was added to the suspension, and the mixture was refluxed for 3 h.The resulting freshly prepared Grignard solution was combined with a solution of methyl phenylacetate (3.0 g, 19.9 mmol) in THF (15 mL) at 0°C.The reaction mixture was then stirred for 24 h at room temperature.After closely monitoring the reaction progress using thinlayer chromatography (TLC), the reaction was quenched by adding a saturated aqueous solution of NH4Cl (50 mL) at 0°C.The aqueous phase was extracted with ethyl acetate (50 mL x 3) and the combined organic layer was washed with a brine solution (50 mL) and dried with MgSO4.The organic phase was evaporated, and the remaining residue was purified using silica gel column chromatography with an eluent mixture of 0-20% ethyl acetate and hexane.This process resulted in the formation of 1,1-bis(4-methoxyphenyl)-2-phenylethanol (3.5 g, 70%) as a white solid.Step 2: A mixture of 1,1-bis(4-methoxyphenyl)-2-phenylethanol (3.2 g, 9.58 mmol) and p-TsOHꞏH2O (0.54 g, 2.87 mmol) in benzene (25 mL) was heated using a Dean-Stark apparatus under reflux conditions for 2 h.Water was then added, and the resulting reaction mixture was extracted with ethyl acetate (20 mL x 3).The combined organic layer was washed with brine (15 mL) and dried using MgSO4.The solvent was evaporated, and the remaining residue was purified through column chromatography, using a mixture of ethyl acetate and hexane (0.5:9.5) as the eluent.As a result, 1,1-bis(4-methoxyphenyl)-2-phenylethylene (2.9 g, 95%) was obtained as a pale-yellow viscous A solution of 6 (2.8 g, 8.86 mmol) in dichloromethane (20 mL) was subjected to the addition of bromine (0.5 mL, 9.74 mmol) at 0°C and stirred for 1 h at this temperature.Then, the reaction was allowed to reach room temperature for 2 h to achieve complete conversion.To quench the reaction, an organic base, Et3N (5 mL), was added at 0°C.The resulting reaction mixture was extracted with ethyl acetate (20 mL x 3).The combined organic layer was washed with brine (25 mL) and dried using MgSO4.The solvent was then evaporated, and the remaining residue was purified by column chromatography using a mixture of ethyl acetate and hexane (1:9) as the eluent.The pure 2,2bis(4-methoxyphenyl)-1-phenylvinyl bromide (7) was yielded as a pale yellow solid (3.1 g, 90%).A solution of compound 7 (1.5 g, 3.8 mmol) and 4-Formylphenylboronic acid pinacol ester (0.96 g, 4.15 mmol) was dissolved in a THF-water (8:2) solvent system, with a total volume of 30 mL.
Potassium carbonate (0.75 g, 5.7 mmol) was added to the solution and degassed for 30 minutes under a nitrogen atmosphere.Tetrakis(triphenylphosphine)palladium Pd(PPh3)4 (0.4 g, 0.38 mmol) was then added and degassed for an additional 10 minutes.The reaction mixture was heated to reflux under nitrogen for 16 h.Once the reaction was determined to be complete by TLC, the mixture was filtered and washed with THF.Water was added, and the resulting mixture was extracted with ethyl acetate twice.The combined organic layer was washed with brine (25 mL) and dried using MgSO4.The solvent was evaporated, and the remaining residue was purified using column chromatography with an eluent mixture ranging from 0% to 50% ethyl acetate and hexane.

Table S8 : The Cartesian coordinates (Z-Matrix) of optimized geometries of the most stable excited state of compound 4.
1 H, 13 C NMR, HRMS, and FT-IR spectra.